K.S.O SISTEM TRANSMISI DIGITAL.

K.S.O

SISTEM TRANSMISI DIGITAL

OVERVEIEW

In this section we develop a simple point-to-point digital

transmission link design considering (Ch8-Keiser):

 Link power budget calculations and

 Link rise time calculations

SIMPLE POINT-TO-POINT LINK

Persyaratan utama sistem link

1. Jarak transmisi yang diinginkan

2. Laju data atau lebar bandwidth

3. BER

This p-p link forms the basis for examining more complex systems

PEMILIHAN PERANGKAT LINK OPTIK DIGITAL

POINT-TO-POINT

No Komponen Jenis Keterangan

1 Serat Optik Single Mode (SM) Ukuran core

Profile indeks bias

Bandwidth atau dispersi Redaman

NA atau Mode-field diameter Multi Mode (MM)

2 Sumber Optik LED Panjang gelombang emisi

Lebar spektral keluaran Daya keluaran

Daerah radiasi efektif Pola emisi

Jumlah mode emisi Laser

3 Detektor Optik PIN Responsivitas

Panjang gelombang operasi Kecepatan respons

Sensitivitas Avalanched Photo Diode (APD)

SYSTEM DESIGN CHOICES:

PHOTODETECTOR, OPTICAL SOURCE, FIBER

Photodetectors:

Compared to APD, PINs are less expensive and more stable with temperature. However PINs have lower sensitivity.

Optical Sources:

• LEDs: 150 (Mb/s).km @ 800-900 nm and larger than 1.5 (Gb/s).km @ 1330 nm

• InGaAsP lasers: 25 (Gb/s).km @ 1330 nm and ideally around 500 (Gb/s).km @ 1550 nm. 10-15 dB more power. However more costly and more complex circuitry.

Fiber:

• Single-mode fibers are often used with lasers or edge-emitting LEDs.

• Multi-mode fibers are normally used with LEDs. NA and should be optimized for any particular application.

SELECTING THE FIBER

Bit rate and distance are the major factors

Other factors to consider:

attenuation and distance-bandwidth product cost of

the connectors, splicing etc.

Then decide

Multimode or single mode

Step or graded index fiber

SELECTING THE

OPTICAL SOURCE

•

Emission wavelength

•

Spectral line width

(FWHM) and number of

modes

•

Output power

•

Stability

•

Emission pattern

•

Effective radiating area

LED

SELECTING THE DETECTOR



Type of detector

 APD: High sensitivity but complex, high bias voltage (40V or more) and expensive

 PIN: Simpler, thermally stable, low bias voltage (5V or less) and less expensive



Responsivity (that depends on the avalanche gain & quantum efficiency)



Operating wavelength and spectral selectivity



Speed (capacitance) and photosensitive area

TYPICAL BIT RATES AT DIFFERENT

WAVELENGTHS

Wavelength LED Systems LASER Systems

800-900 nm (Typically Multimode Fiber) 150 Mb/s.km 2500 Mb/s.km 1300 nm (Lowest dispersion) 1500 Mb/s.km 25 Gb/s.km (InGaAsP Laser) 1550 nm (Lowest Attenuation) 1200 Mb/s.km Up to 500 Gb/s.km (Best demo)

DESIGN CONSIDERATIONS

Link Power Budget

 There is enough power margin in the system to meet the given BER

Rise Time Budget

 Each element of the link is fast enough to meet the given bit rate

These two budgets give necessary conditions

for satisfactory operation

RECEIVER SENSITIVITIES VS BIT RATE

Sensitivitas penerima sebagai fungsi laju bit

OPTICAL POWER-LOSS MODEL

Keterangan:

𝛼f : Konstanta redaman fiber [dB/Km]

lc : Loss konektor [dB]

LINK POWER BUDGET

Loss daya total:

Dimana:

PT : Loss daya total [dBm]

PS : Daya optik dipancarkan dari sumber ujung fiber [dBm]

PR : Sensitivitas detektor [dBm] m : (Jumlah) konektor n : (Jumlah) splicer L : Panjang link [Km] System margin [dB]

ystem Margin

T s R c sp f

P

 

P

P



ml



nl





L



S

LINK POWER BUDGET TABLE

Example:

[SONET OC-48 (2.5 Gb/s) link]

Transmitter:

3dBm @ 1550 nm;

Receiver:

InGaAs APD with -32 dBm

sensitivity @ 2.5 Gb/s;

Fiber:

60 km long with 0.3 dB/km

attenuation; jumper cable loss 3 dB

each, connector loss of 1 dB each.

Component/loss parameter Output/sensitivi ty/loss Power margin (dB) Laser output 3 dBm APD Sensitivity @ 2.5 Gb/s -32 dBm Allowed loss 3-(-32) dBm 35 Source connector loss 1 dB 34 Jumper+Connecto r loss 3+1 dB 30 Cable attenuation 18 dB 12 Jumper+Connecto r loss 3+1 dB 8 Receiver Connector loss 1 dB 7(final margin)

RISE TIME BUDGET

Untuk menentukan pembatasan dispersi link fiber optic

. Total rise time depends (tsys) on:

 Transmitter rise time (t_tx)

 Group Velocity Dispersion (t_GVD)

 Modal dispersion rise time (t_mod)

 Receiver rise time (t_rx)

 Rise time contributor (ti)

1/ 2 2 1 n sys i i

t

t







 

_





_

Total rise time of a digital link should not exceed

RISE TIME BUDGET

Umumnya degradasi transition time link digital:

•NRZ  ≤ 70% perioda bit

•RZ  ≤ 35% perioda bit

Respon front end penerima dpt dimodelkan sbg LPF orde pertama: Brx : lebar pita elektrik 3 dB dr penerima

u(t) : fungsi tangga berharga 1 utk t ≥ 0 dan 0 utk t < 0 Rise time penerima (10 % - 90 %)g(t) :

trx : dlm ns

PENGKODEAN SALURAN

Format sinyal optis transmisi penting utk dipertimbangkan krn kepraktisan, sirkit decision hrs dpt memisahkan secara tepat informasi timing.

Maksud timing :

(a) Memungkinkan sinyal disampling pd S/N maks (b) Menjaga spasi pulsa

(c) Menunjukan interval start dan stop/end

Pengkodean sinyal menggunakan sejumlah aturan utk mengurutkan simbol sinyal dgn pola tertentu.

Jenis dasar kode saluran biner dua-level pd trans optik : (a) NRZ

(b) RZ

KODE NRZ

- Mudah dibangkitkan/dikodekan - Mudah di-dekodekan

- Tdk memiliki error monitoring atau kemampuan koreksi - Ttdk memiliki self-clocking (timing)

- Lebar pita minimal

- Daya rata masukan penerima tergantung pd pola data  base line wander

KODE NRZ

KODE RZ

- Tiap data bit dikodekan dgn dua bit kode saluran

- Unipolar  string 0 panjang akan kehilangan sinkronisasi timing - Biphase  timing dpt diatasi

KODE RZ

KODE BLOK

- Kode blok mBnB (n > m) : tiap m bit biner dikodekan dgn n bit biner. - Peningkatan lebar pita sebesar n/m

- Timing cukup

- Terdpt informasi error minitoring

PERBANDINGAN BEBERAPA KODE MBNB

W : Pesentase n-bit word yg tidak digunakan Nmax : Jumlah simbol identik berurutan terpanjang D : Batas disparitas terakumulasi

LATIHAN

Rancangan siskom optik laju data 60 Mb/s sbb : Jarak 60 Km

Fiber SM konstanta redaman 0,2 dB/Km, pelebaran pulsa dispersi material 2 ps/Km, panjang kabel 2 Km/haspel.

Redaman splice 0,2 dB/bh Redaman konektor 0,5 dB/bh

Sumber : daya 1 mW, rise time 5 ns

Detektor : sensitifitas – 40 dBm (BER 10-9), rise time 2 ns Margin sistem = 6 dB

Selidiki apakah sistem tsb memenuhi anggaran daya ?

Selidiki apakah sistem tsb memenuhi anggaran rise time transmisi NRZ dan RZ ? Kesimpulan ?

LATIHAN

Suatu siskom optik memiliki spesifikasi :

λ = 1,3 μm trx = 0,35 ns

B = 1 Gb/s Dmat = 2 ps/(Km-nm)

Fiber SM panjang kabel 2 Km/haspel

αf = 0,4 dB/Km lsp = 0,1 dB/bh

σλ = 3 nm lc = 1 dB/bh

ttx = 0,25 ns Ms = 6 dB

Ps = 1 mW Pr = - 42 dBm (BER 10-9)

L = 60 Km twg diabaikan

RISE TIME BUDGET

M Hz

in

bandwidth

receiver

is

where

ns;

350

rx

B

/B

t

_rx



_rx Similarly

tx

tx

B

t



350

/

ns

Assuming both transmitter and receiver as first order

low pass filters

MODAL DISPERSION RISE TIME

Bandwidth B_M(L) due to modal dispersion of a link length L is empirically given by,

B₀ is the BW per km (MHz-km product) and q ~0.5-1 is the modal equilibrium factor q o M

L

B

L

B

(

)



/

(ns)

/

440

/

44

.

0

₀ mod

B

L

B

t



_M



q

GROUP VELOCITY DISPERSION

  L D t_GVD | |

Where,

D is the dispersion parameter (ns/km/nm) given by eq. (3.57)

σ

_λ

is the half power spectral width of the source (nm)

L is the distance in km





L

D

t

_GVD



|

|

TOTAL RISE-TIME

2 / 1 2 2 2 2 2 0 2 2 / 1 2 2 2 mod 2

350

440

]

[

























































rx q tx rx GVD tx sys

B

L

D

B

L

t

t

t

t

t

t





source the of width Spectral : [nm] Dispersion : )] . /( [ dispersion velocity group to due time -rise : [ns] 7 . 0 fiber; the of km 1 the of : ] [ fiber the of Length : ] [ BW Electrical 3dB : ] [ dispersion modal : ] [ time rise receiver : ] [ time rise er transmitt : ] [ 0 mo d   nm km ns D t q BW MHz B km L MHz B n t ns t ns tx t GVD rx rx 

EXAMPLE: TRANSMISSION DISTANCE FOR

MM-FIBER

NRZ signaling, source/detector: 800-900 nm LED/pin or AlGaAs laser/APD

combinations. ; LED output=-13 dBm;fiber loss=3.5 dB/km;fiber

bandwidth 800 MHz.km; q=0.7; 1-dB connector/coupling loss at each end; 6

dB system margin, material dispersion ins 0.07 ns/(km.nm); spectral width for

LED=50 nm. Laser ar 850 nm spectral width=1 nm; laser ouput=0 dBm, Laser

system margin=8 dB;

9

10

PARAMETERS FOR FIG 8-5

Power coupled

from LED : -13

dBm

Fiber loss 3.5

dB/km

System Margin 6

dB, couplers 1dB

(LED-PIN)

D

_mat

= 0.07

ns/(nm.km)

LED  50 nm

LASER 1 nm

Bo=800 MHz-km

q = 0.7 (modal)

Power coupled

from LASER = 0

dBm

Material

dispersion limit

with LASER is

off the graph

System Margin 8

dB (Laser-APD)

EXAMPLE:TRANSMISSION DISTANCE FOR A

SM FIBER

Communication at 1550 nm, no modal dispersion, Source:Laser;

Receiver:InGaAs-APD (11.5 log B -71.0 dBm) and PIN (11.5log B-60.5 dBm); Fiber loss =0.3 dB/km; D=2.5 ps/(km.nm): laser spectral width 1 and 3.5 nm; laser output 0 dBm,laser system margin=8 dB;

SYSTEM RISE-TIME & INFORMATION RATE

In digital transmission system, the system rise-time limits the bit rate of the

system according to the following criteria:

period

bit

RZ

of

%

35

period

bit

NRZ

of

%

70





sys

sys

t

t

EXAMPLE

Laser Tx has a rise-time of 25 ps at 1550 nm and spectral width of 0.1 nm. Length of fiber is 60 km with dispersion 2 ps/(nm.km). The InGaAs APD has a 2.5 GHz BW. The rise-time budget (required) of the system for NRZ signaling is 0.28 ns whereas the total rise-time due to

ANALOG COMMUNICATION LINKS

Analog (RF) links are used in

Analog TV and audio services (Legacy) Cable modem services

MULTI CHANNEL SYSTEMS

Number of RF carriers can be summed and directly modulate the laser

MULTI CHANNEL SYSTEMS

These have the capability to multiplex several RF channels

Each RF channel is independent, it may carry different type of data (analog video, digital video, digital audio etc.)

The data could be modulated onto the RF carrier using different techniques (AM, FM, QAM etc.)